1. Field of the Invention
The present invention relates to an electrically assisted vehicle such as an electrically assisted bicycle and wheelchair which has a driving power applying section for applying human driving power to drive a driving wheel as well as an electric driving section for driving an electric motor to auxiliarily drive the driving wheel in correspondence with the human driving power applied to the driving power applying section.
2. Description of Related Art
Generally known electrically assisted vehicles of this kind include electrically assisted bicycles. In an electrically assisted bicycle, the human driving power applied on pedals is supplemented with electric driving power by an electric motor constantly at an assist ratio of 1:1. The assist ratio is a ratio of the electric driving power to the human driving power. Thus, the electrically assisted bicycle needs only half of the human driving power that is required for driving a bicycle which is driven only by human power.
In the conventional electrically assisted bicycles, only one mode is provided for controlling the assist ratio. For example, the assist ratio is predetermined as shown in FIG. 6 and the electric driving power is outputted according to this predetermined assist ratio. In this figure, a torque of the human power is plotted in abscissa and the assist ratio of the electric driving power is plotted in ordinate. This figure shows at what assist ratio the electric driving power is outputted when a certain value is inputted as the human power torque. When the human power torque is small, for example, when the bicycle is traveling on level ground, the assist ratio is set to be small because much assistance by the electric driving power is not required. The assist ratio increases gradually as the human power torque increases. When the human power torque is large, for example, when the bicycle is being started or is traveling up a slope, much assistance by the electric driving power is required. Accordingly, when the human power torque exceeds a predetermined value, the output of the motor is set to equal the human power torque.
In this case, the consumption of a battery is saved for long battery life by setting a small assist ratio for a small human power torque.
Though the above-described control is suitable for saving the consumption of the battery, a problem occurs in working pedals, for example, when traveling up a slope or starting the bicycle.
FIG. 14 illustrates the problem. In FIG. 14, the torque is plotted in ordinate and time is plotted in abscissa. A curving solid line represents changes in the power applied to the pedals, that is, changes in the human power torque. A curving dotted line represents changes in the output torque by the motor. A straight solid line represents an average human power torque, and a straight dotted line represents an average torque outputted by the motor. This figure shows that, since the assist ratio of the output by the motor to the human power torque is set as shown in FIG. 6, the torque outputted by the motor is small when the human power torque is small, and that there is a difference between the average human torque and motor torque. In other words, a user sometimes feels that he/she does not obtain enough torque where the assist ratio is small, for example, while traveling up a slope.